Diaphragmless microphone



E. REISZ.

DIAPHRAGMLBSS MICROPHO NLv June '28 1927.

Filed. Nov. 'l2'.F 1925 2 Sheets-Sheet 2 EUGEN :QE/sz BY Y.

l ,ATTORNEYS Fetenteai une 2te, we?.

.lleettcn tied Bcvczeter 12, iets', We. w, and in Germany September 1u,1925.

clay and certain liquids or seits into e piestio mess on which the soundwaves ci the eir ect,

These microphones do not hcwever possess any advantage, over thediaphragm micro i0 phones because their sensitiveness over e range offrequencies of from 100 to 7000 vibrations per seccn (such as `isnorrneiiy required) is not constant, that is to eey, even in thesearrangements denite frequencies accentuated. This isnt is shown by,

'the oiiowing consideration.

'Wien cosi dust is useci without u binciinej ugent the speces betweenthe grains ci coal form a passage for the soun@l waves enti, moreespecieiiy, for the sound weves with low frequencies, so that the.sensitivencss increases with the irecguenc?,Y and 'then tiecreeses againowingi to the deieterious shunt eiect of the iower eyers. "When :1binding agent is used, the purticies of coei no ionger oscilieteincpendenty. oi each other hut form a coherent osciieting mess whichmust 'possess resonance points in exectiy the seme we)7 es a carbondiaphragm.

According to the present invention, these disadvantages are overcome hyusing a mirture of coarse enci moiecuieriy fine cosi dust as e iiiiingmateriel and piiingr this mixture into a. vibration-free receptacie tosuch e height that the sound waves oi'y the lowest frequency occurringrure coninieteiy ahsorhec'i in the iiliing. the WhoisA ot thesoundtenergyv being thus showed to exert its pressure aetion.

Figure i shows e :iront View of the inicrophone with theinsuiatingg filmcovering the carbon chambers, the frames supporting; the

l iim and the nridplete remove.

Figure 2 shows a horizontei section of the microphone.

Figure 3 showsin :i conventioneirepresentation, the arrangement or thecarbon grains and the two electrodes. m`

Figure i is c. characteristic curr/e showing the proportion of thefrequencies of the sound weves to the amount of the change ofresistance.

Figure 5 shows a vsimiier curve for varied heights of the piie ofcer-hon grains. `--"F:i,g'ure 6 shows :i similar curve for varied sizeci' fthe powder.

Figure 7 shows e. curve for e low height of the cerhon powder for Ahighfrequencies.

Before describing e, constructional form of apparatus eccorfiing to 'thepresent invention, I wiil first explain, with reference to Figures 3 toci the accompanying drawings, the operation of a fiiayrlhragmlessmicrophone with e Contact powder tilting.

He Vibration-free receptece (see Figure f3) he teken, in which twoiixeci electrodes are provide@ for the supply of current and coal dustbe spread inthe-chambers between these eiectrocies, s microphone 'willhe obtained the grains of the 'eerhon dust or operation of which isgrephimilly` represented hy curves es in 1ffigures 4 and 5. For e,`given size of csrhon powder granule and when powder is p'ilefi up inthe chamber to u certain height the characteristic curve shown in Figure4i is obtained. Along the ebscisse erel set out the periodicities of thesound waves which excite the microphone und n rions; the ordinate, theksensibility veines, that to soy, the magnitudes of the veristions ofresistance. If the height to which the 'powder is piieri up he Varied,the dotted series ci curves (see Figure 5) will he ohtniuefi according?to the increase in the thickness of he iep-er ci' granules, while a.fiecrecse such thickness Wiil he represented hy the riot und 'dashcurves.

A simiicr fiispizicement of the curves also taires piece, as showninFigure 6, if' the size ot the freins of the carbon 'powcier he variedenti, with an increased size of grenuie the originai curve i is changedinto curve 2.

These phenomene, which are also confirmed h3 exhaustive experimentswhich the applicant has marie are explained hylgthe fact that thesoi'end Waves with the lower frcquencies e great extent through theinterstitial speces between the cerhon pow fier granules and, therefore,do not produce :inv Variation of pressure on the grains themseives. 0nthe other hsnfhthese interstitial spaces orier a very high resistancethe sound waves with the higher frequencies, so that these sound wavesare completely absorbed 'even the uppermost ieyers of powder and theiower layers then constitute a deleterious shunt to that port-ion of thecarhon powder which is sensitive to changes of resistance. Thus, in therange oi' musical frequencies,

menare lwhich extends to 7000 vibrations per second, 1t 1s evidentlynecessary that the carbon powder vbe piled into the granule chamber to avery small height only. The obtention 'effected by using material havinggranules of various diderent sizes, varying say from about 1 mm. down tothe very finest pulverulent molecular powder, because it is onlyV withsuch exceedingly fine material that the l powder c.

interstitial spaces can be completely closed agamst passage of the soundwaves. `When `such material is used as Contact powder for theconstructional form of the microphone shown in Figure 3, there results acharacteristie curve, shown in Figure 7, which runs in a straight lineover a sutliciently wide range of frequency.

The same effect could be obtained by the use of very tine powder alonebut for the 'fact that such powder has the property of caking together,thus rendering the desired individual oscillation of the separate grainsillusory.

'In the constructional form of the microphone shown in Figures l and 2,the layer of carbon is arranged vertically on the front of a fixedvibration free body e. g., a block a of marble. The fixed electrodes ofcarbon -or of a non-oxidizable metal, which form the connection Vbetweenthe carbon powder and the conductor wires (not shown) are fixed in twodeep troughs 71. in the marble block a, in such a way that their freeexternal surfaces are entirely covered by the contact The attachment iseffected for vexample by providing the electrodes b with screws o winchare screwed into heads c of the screw belts-z' which heads are conformedto the shape of the electrodes. These bolts z' pass through the` bodya'and are held on the reverse side thereof by nuts Z which Vservie atthe same time, to carry the current.

The two lateral deep troughs fr merge at the front of the body a intothe granule chamber y, which is exposed to action or the sound waves andis made very fiat, so that the layer of powder may be equally sensitiveto variations of pressure for all the various frequencies that occur.The entire chamber is covered in at the front by a thin skin f ofrubber, which keeps the powder under a fixed pressure when the chamberis in a vertical position.

The use of a rubber skin for preventing the carbon granules from fallingout is known, but in the arrangement herein described it serves afurther purpose tor, by

reason of its stretched condition, it increases the pressure under whichthe contact grains are maintained and makes the mass oi powder perfectlysound proof even when a, very thin layer is used. This rubber skin isheld by a grid plate n (of fabric, gauze or the like) provided with aprojecting rim m.

It is especially important also that the fixed electrodes shall not beattacked "by the sound waves; otherwise the Velectr-mie rods begin tooscillate. whereby resonance effects are produced in the system.

In the practical constructional form shown in Figure l, there isarranged-above the actual granule chamber, a subsidiary chamber p whichcan be closed bymeans of a plug g and the purpose of which is to supplythe amount of powder required to keep up the constant pressure when thecontact powder in the main chamber `settles down with lapse of time.

l claim:

l. In a microphonma vibration free insulating base, a pair of fixedelectrodes mounted therein. a layer oi' Contact material extendingbetween and over said fixed elcctrodes, said contact material consistingot a. mixture ot' coarse grained and fine dust or powder filling 4theinteisticcs lbetween the grains and flexible `means for holding andpressing the grains together withia constant normal pressure.

2. In a microphone, a vibration free insulating base havingla chamber. a.pair of fixed electrodes mounted thcreinnin the base, a layer of`contact material in the chamber and extending o'rer said xedelectrodes, Said contact materia/l consisting ot' a mixture of granulesand fine dust or powder filling the intcrstices between the granules'and a flexible stretched skin of an insulating material extending oversaid layer of contact material and' holding the same in said chamberwith a constant normal pressure.

3. ln a microphone, a vibration free insulating base, provided with achamber having depressions at opposite ends, a pair of elongated fixedelectrodes in the depressions of the said base, a layer of Contactmatcrial in the chamber and extending between and into said depressions,so as topartially surround said electrodes, said Contact materialconsisting of a mixture of granules lao'l llating material extendingover saidfilling,

and a 'grid secured to the base and securing the skin in position.

5. In a microphone, a vibration-free in'- su'lating base, a granulechamber formed therein, trough-like recesses in the bottom of saidgranule chamber and extending across opposite ends thereof, elongatedelectrodes housed in said trough-like recesses, electrical conductinmeans extending through said base to said fixed electrodes, a layer ofcontact material filling sa-id granule chamber and extending into saidrecesses and partly around said hxed' electrodes, a iiexible skin of aninsulating material stretched over said granule chamber, and meanswhereby the chamber is kept filled.

6. In a microphone, a 'vibration-free in sulatingl base, a granulechamber formed therein, trough-like recesses in the bottom' of saidgranule chamber and extending across opposite ends thereof, elongatedelectrodes housed in said trough-like recesses, electrical conductingbolts extending through said base and having heads fitting u on andsecured to the electrodes, a contact lling in the granule chamber andthe trough-like recesses, and

partly surrounding the electrodes, flexible means arranged to close saidgranule cham ber, and'a grid plate having a rim and secured to the baseand securing the exible means in place.

7, In. a microphone, a vibration-free insu.

a granule chamber formed therelating base in, trough-like recesses inthe bottomv of said granule'chamber and extending across opposite .endsthereof, fixed electrodes housed in said trough-likerecesses,'electricalconducting means extending through said base to saidfixed 'electrodes a granular contact fillin for the granule chamber andthe troug -likerecesses, said filling consisting of a mass of coarsegranules having their interstitial spaces completely filled bymolecularly fine powder and iiexible means arn ranged to close saidgranule chamber and to exert a constant normal pressure on said 'filllngBIn a microphone, a vibration-free insulating base', a granule chamberformed therein, trough-like recesses in the bottom of said granulechamber and extending across opposite ends thereof, fixed electrodeshoused in said trough-like recesses, electrical conductin meansextending through said base to sai fixed electrodes, a layer of contactpowder filling said granule chamber and extendinginto said recessesand'around said fixed electrodes, flexible means arranged to close saidgranule chamber and to exert a constant normal pressure on saidfilling,.an

and extending into said recesses and around said iixedelectrodes,iexible means' arranged to close said granule chamber and to exert aconstant normal pressure on said filling, an auxiliary contact powderchamber in sai-d insulating base, an opening from said auxiiiary chamberinto the granule chamber, an openingl from saidauxiliary chamber to anexterior surface of the base and means` for closing said last namedopening.

In testimony whereof I have signed name to this specification. l

vwenn Brisa i

